There are various methods for optically measuring a sample and obtaining the characteristics of the sample from the measurement results. One such method optically measures a sample and obtains the coagulation time based on the measurement results. This method uses, for example, blood plasma as a sample to which predetermined reagent is added, and the change in turbidity which occurs in conjunction with the coagulation of the plasma is measured as the change in the amount of transmission light and the amount of scattered light to obtain the blood coagulation.
Japanese Laid-Open Patent Publication No. 10-123140 discloses a blood coagulation analyzer which uses such an optical method. The blood coagulation analyzer radiates light on a blood sample within a transparent container to which reagent has been added, and calculates the coagulation time based on the coagulation saturation value (coagulation reaction end point) from the change in the amount of scattered light over time.
Specifically, the measured value of the amount of scattered light is input to a measuring unit at predetermined time intervals, the latest input value after the coagulation reaction has started is compared to an input value from prior to the predetermined time interval, and the latest input value is set as a provisional saturation value when the difference between the two values (that is, the predetermined time interval (unit time) per amount of change in the input value) is less than a predetermined value (threshold value). Subsequently, the provisional saturation value is set as the true saturation value when there is no further change in the provisional saturation value, and the time to achieve 50% of this change is determined as the coagulation time when change from the amount of scattered light at the start of the coagulation time to the amount of scattered light at the true saturation value is deemed as 100%.
Japanese Laid-Open Patent Publication No. 6-249855 also discloses a blood coagulation analyzer which uses such an optical method. The blood coagulation analyzer radiates light on a blood sample within a transparent container to which reagent has been added, and calculates the coagulation time based on the ratio of the integration values at short intervals of the A/D-converted scattered light data.
Specifically, A/D conversion data obtained by measuring the amount of scattered light are smoothed and the origin point adjusted then set as basic A/D conversion data; the basic ratio data are then calculated, that is, the ratio of integration values of the integrated basic integration data and basic A/D-conversion data at adjacent short intervals is calculated; basic A/D conversion data values are selected to determine the coagulation time from the time at which the basic ratio data reaches preset fixed basic ratio data; and then the time corresponding to the basic A/D conversion data value 1/N (where N is a constant integer of 1 or higher) is determined from the mixing time and set as the coagulation time.
Since the saturation value is determined based on the difference between two input values in the art disclosed in Japanese Laid-Open Patent Publication No. 10-123140, an accurate saturation value can not be determined because there is a large fluctuation in the difference between the two input values and the noise in each input value. Moreover, in some cases of specific measurement items and blood samples (for example, samples with high concentration of fibrin or heparin) which exhibit constant change in the amount of scattered light even after the actual coagulation reaction has ended, it may be difficult to determine the point at which the difference between the two input values becomes less than the threshold value. Thus the determination of the saturation value itself becomes quite difficult.
Furthermore, the art disclosed in Japanese Laid-Open Patent Publication No. 6-249855 is easily affected by fluctuations in the amount of scattered light in the short intervals since the coagulation time is calculated based on the basic ratio data, which is the ratio of the integration values of the basic integration data and the basic A/D conversion data at adjacent short intervals.